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Abstract

Heavy metal concentrations and magnetic susceptibility of sediment samples were analyzed
as indicators of urban and industrial contamination in Anzali wetland in Gilan, Iran.
The aim was to investigate the suitability of magnetic properties measurements for
indicating heavy metal pollution. The concentration of six heavy metals (Ni, Cr, Cd,
Zn, Fe, and Pb) was determined in different depths of four sediment core samples within
four different regions of the wetland (Abkenar, Hendekhaleh, Shijan and Siakeshim).
Average concentration of heavy metals in the sediment cores was higher than the severe
effect level (SEL) for Ni, Cr and Fe (77.26, 113.63 ppm and 5.2%, respectively) and
lower than SEL for Cd, Zn and Pb (0.84, 137.7, 29.77 ppm, respectively). It was found
that the trend of metal concentrations with the depth is different in each core and
is related to the pollution discharges into the rivers entering the wetland. Core
magnetic susceptibility measurements also showed different magnetic properties in
each core. Cluster analysis was applied using Pearson correlation coefficient between
heavy metal concentrations and magnetic properties across each core. Significant relationship
was found to exist between magnetic susceptibility and the concentration of Ni in
Abkenar and the concentration of Fe in other regions. Whereas Abkenar is almost the
isolated and uncontaminated region of the wetland, it revealed a difference in magnetic
properties between contaminated and uncontaminated sediments. It was concluded that
magnetic properties of samples from contaminated zone were mostly related to Fe content.
The result of this study demonstrated that magnetic susceptibility measurements could
be applied as a proxy method for heavy metal pollution determination in marine environments
in Iran especially as a rapid and cost-effective introductory site assessments.

Keywords:

Introduction

Magnetic susceptibility (MS) measurements are being used as an approximate tool for
detecting industrial pollutions, because they are comparatively simple, fast and cost
effective analyses. [1-7]). This method also could be applied as a tool for the assessment of heavy metal contamination
in sediments [8], on the other hand as a proxy for heavy metal pollution. Petrovsky′ and Ellwood [6] discovered that magnetic susceptibility and Zn concentrations show very similar spatial
distributions in a 20,000 m2 area at the Litavka River, Czech Republic, where ashes from a lead smelter are weathering
in the fluvisols. Chan et al., [2], revealed that a significant correlation exists between the magnetic susceptibility
and the concentration of Pb, Zn and Cu as well as Tomlinson pollution load index (PLI)
in seabed sediments of Hong Kong Harbour. Schmidt et al., [9], investigated the suitability of field magnetic measurements for indicating heavy
metal pollution. Geochemical analysis of their soil samples from Bradford, England,
showed close correlation of concentrations between Fe, Cu, Mn, and Ni. In addition,
Fe concentrations correlated with magnetic susceptibility field measurements. The
results of their study demonstrated the potential of magnetic susceptibility field
mapping for fast preliminary site assessment, greatly reducing the scale of subsequent
geochemical sampling and analysis.

Magnetic susceptibility measurements, in Iran, have been applied for survey on Caspian
sea-level fluctuation [10], but as a proxy for industrial contamination has been employed only in urban topsoils
in the arid region of Isfahan [5]. They measured the magnetic susceptibility of 113 collected soil samples from public
parks and green strips along the rim of roads with high-density traffic within the
city of Isfahan using the Bartington MS2 dual frequency sensor. As, Cd, Cr, Ba, Cu,
Mn, Pb, Zn, Sr and V concentrations were also measured in all collected soil samples.
They discovered that Pb , Cu, Zn, and Ba have positive significant correlations with
magnetic susceptibility, but As, Sr, Cd, Mn, Cr and V have no significant correlation
with the magnetic susceptibility. There was also a significant correlation between
pollution load index (PLI) and magnetic susceptibility. Finally they indicated the
potential of the magnetometric methods to evaluate the heavy metal pollution of soils.

The present study is trying to investigate the suitability of magnetic properties
measurements for indicating heavy metal pollution in Anzali wetland at the north of
Iran. The result of this study suggests a useful, fast and cost-effective method for
assessment of environmental pollutions in Iran.

Materials and methods

Study area

Anzali wetland is a large complex environment of fresh water lagoons with extensive
reed-beds, shallow impoundments and seasonal flooded meadows. It is extremely important
as a spawning and nursery ground for fish, and as a breeding, staging and wintering
area for a wide variety of waterfowl. It is located in the northern part of Iran,
along the coast of the Caspian Sea approximately at north latitude between 37o 25′ and 37o 32′ and east longitude between 49o 15′ and 49 o 36′. It has a catchment area of 3610 km2. Approximately 42% of the catchment area is
covered by forests. Among the landuse categories, forest has the largest share of
42%, followed by paddy field/farmland (26.7%) and orchard (8.6%) in that order (Figure 1). There are 10 major river systems entering the wetland and some of them have large
discharges of urban and industrial wastewater along their way. The annual mean discharge
into the wetland is estimated at 76.14 m3/s and the average COD is about 26.5 mg/L. The average annual precipitation of Anzali
wetland watershed is about 1200 mm. Considering inflowing rivers, the wetland can
be divided to 2 zones (Figure 2): west region with only one river inflow (Zone A), and the other regions with 9 rivers
inflows (Zone B).

Sampling locations

The Anzali wetland is shaped from 4 regions: west region (Abkenar), central region
(Hendekhaleh), Siakeshim and east region (Shijan). Sampling sites were chosen approximately
in the center of each region to represent the situation of each part (Figure 2): St1 in Abkenar, St2 in Hendekhaleh, St3 in Shijan and St4 in Siakeshim. Situation
of sampling point in some locations (low depth marsh areas) was also considered for
ease of sampling.

Sampling methods

Sediment cores were collected in slosh mode using a piston gravity corer in May 2011.
The core lengths were 70, 70, 80 and 50 cm and the diameter was 6 cm. All the samples
were sealed by nylon and transferred to sediment laboratory of Iranian National Institute
for Oceanography, Tehran, Iran, for magnetic susceptibility analysis. For geochemical
analyses, they were transferred to sediment and chemistry laboratory of Water Research
Institute, Tehran, Iran. After the polyethylene tube was cut off carefully, the sediment
columns were sectioned into slices in depths of 0, 2, 6, 10, 15, 30, 50 and 70 cm
along core 1 and core 2; 0, 2, 6, 10, 15, 20, 40, 60 and 80 cm along core 3 and 0,
2, 6, 10, 15, 30 and 50 cm along core 4.

Magnetic susceptibility measurements

Magnetic susceptibility (MS) is a measure that particular sediments are magnetized
when subjected to a magnetic field. The ease of magnetization is ultimately related
to the concentration and composition (size, shape and mineralogy) of magnetizable
material contained within the sample. Any sediment core possessing downcore variation
per unit volume in the concentration and composition of magnetizable minerals will
yield a MS curve reflecting these changes [11].

Magnetic susceptibility measurements are a non-destructive and cost effective method
of determining the presence of iron-bearing minerals within the sediments. The whole
cores, or individual sediment samples, are exposed to an external magnetic field which
causes the sediments to become magnetized according to the amount of Fe-bearing minerals
present in the samples.

In our system, using Bartington MS2 System [11] whole cores are moved incrementally (generally in 1 cm) by a track motor through
a susceptibility loop (of varying size) in which a magnetic field is generated and
which magnetizes the sample susceptible substances (minerals or mineraloids) within
the sediment. Samples that are rich, per unit volume, in magnetizable substances will
yield high readings.Samples that are poor in magnetizable substances, or contain diamagnetic
minerals, will yield lower or negative values.

Geochemical analysis

Subsamples for geochemical analysis were chosen incrementally in different depths
along core samples, dried and powdered in agate mortar. Digestion of organic matter
and dissolution of silicates for total elemental analysis were done as described below:
1.0 g of the 100-mesh (0.15 mm) sediment was weighed into a 100-mL Teflon beaker and
10 mL of HNO3 and 10 mL of HClO4 were added. The beaker was covered with a Teflon watch cover and heated at 200°C
for 1 h. The cover was removed and heating was continued until the volume became 2
to 3 mL. After cooling the sample, 5 mL of HClO4 and 10 ml of HF were added; Teflon cover was put and heated at 200°C until all siliceous
materials had been dissolved. Then the cover removed and heating continued until the
volume was 2 to 3 mL. The digest was cooled, 10 mL of 50% HCl was added, Teflon cover
put and heated at 100°C for 30 min. After cooling the sample brought to 50-mL volume.
The solution is then ready for ICP determination [12]. The concentrations of heavy metals were determined by Varian 710-ES Inductively
Coupled Plasma Mass Spectrometry (ICP-MS) according to APHA AWWA, WEF [13]. Each sample was duplicated and the average was reported.

Pollution assessment

To assess metal concentrations in sediment, the New York State Department of Environmental
Conservation [14] guideline was applied. It proposed the lowest effect screening levels (LEL) for Ni,
Cr, Cd, Zn, Pb, and Fe of 31, 26, 0.6, 120, 31 mg/kg and 2%, respectively, and severe
effect screening levels (SEL) of 75, 110, 9, 270, 110 mg/kg, and 4%, respectively.
The pollution extent was assessed by two threshold values of LEL and SEL. If the LEL
was exceeded, the metal could moderately impact biota health. If the SEL was exceeded,
the metal could severely impact biota health [15].

Results

Geochemical results

Table 1 shows the concentration of heavy metals in subsamples in different depths and the
average value of each core. Total average values of Ni, Cr, Cd, Zn, Pb, and Fe were
76.91, 113.74, 0.84, 137.98, 29.74 ppm and 5.2%, respectively. Comparison of the average
value of heavy metal concentrations in sediment cores is illustrated in Figure 3. It can be observed that the values related to core 1 is lower than the other cores.
Whereas core 1 represents Abkenar region, it could be resulted that this part of the
wetland is less contaminated than the other parts. This fact could be deduced from
rivers entering the wetland. As described in section 2.1 (Figure 2) only one river enters the west part of the wetland and the other rivers which are
carrying urban and industrial wastewaters enter the other parts of the wetland.

Figure 3.Comparison between average values of heavy metal concentrations in sediment cores.

Nickle contamination

The average value of Ni concentration was above SEL (50 mg/kg) at all cores. The maximum
Ni concentration appeared at the depth of 50 cm at core 4 (Siakeshim), which was more
than two times the SEL. A relatively constant Ni concentration was detected across
core 1 (Abkenar), but in core 2 it increases with the depth increase to concentration
of 105 ppm at the depth of 50 cm and decreases to 98 ppm at the depth of 70 cm. The
minimum value of Ni concentration appeared at the depth of 15 cm in core 3.

Choromium contamination

At core 2 and core 3 (Hendekhale and Shijan), the average value of Cr concentration
was above SEL (110 mg/kg) and the maximum value appeared at the depth of 2 cm at core
2. In core 1 and core 4, the average Cr concentrations were below SEL and above LEL
(26 mg/kg). The minimum value of Cr concentration appeared at the surface of core
1.

Cadmium contamination

All of the Cd concentrations were below SEL (9 mg/kg) but the average values of Cd
concentration in core 2 and core 3 were above LEL (0.6 mg/kg). The maximum concentration
appeared at the surface of core 3 and the minimum value appeared at core 1.

Zinc contamination

Zn concentration in all subsamples was below SEL (270 mg/kg) except for core 2 at
the depth of 2 cm. the average value of Zn concentration was near LEL (120 mg/kg)
in core 1, core 2 and core 4 and the minimum value appeared at the surface of core
1.

Lead contamination

The average concentration of Pb in all sediment columns was below LEL (31 mg/kg) except
for core 3 (Shijan), which was above LEL and below SEL (110 mg/kg). A relatively constant
Pb concentration was detected across core 1 (Abkenar) but in core 4 it increases with
the depth increase. The maximum value of Pb concentration appeared at the depth of
15 cm in core 3 and the minimum value appeared at the surface of core 4.

Iron contamination

All of the average values for Fe percentage in sediment columns were above SEL (4%)
and the maximum value of appeared at the depth of 10 cm in core 4. A relatively constant
Fe concentration was detected across core 1, core 3 and core 4.

Figures 4,5,6,7 show the variation of heavy metal concentrations across core 1 to core 4 respectively.
It can be observed that there is no distinct trend for concentration with the depth
in sediment columns especially for core 2 and core 3. In core 1 as illustrated in
Figure 4, the concentration of Pb, Cr, Zn and Cd is increasing first, and then decreases through
the core depth. Fe and Ni concentrations have a relatively constant variation with
the depth. In core 4, the concentration of Pb, Ni and Cr have increasing variation
with the depth. Zn concentration decreases to the depth of 30 cm and increases to
the end of the column. These variations are related to contaminations which have entered
to the wetland during recent years.

Magnetic susceptibility results

Magnetic susceptibility curves (MS curves) of core 1 to core 4 are illustrated in
Figure 8. Variation of magnetic susceptibility with depth in core 1 is increasing to the middle
depth and decreases to the end of the core. In core 2 magnetic susceptibility increases
to depth of 25 cm and decreases to the end. Core 3 has three peaks in depths of 10,
45 and 60 cm and finally core 4 has two peaks in depth of 10 cm and 35 cm.

Discussion

Table 2 compares the results of this study with previous studies on the Anzali wetland and
some other lakes in the world. It should be explained that the values in this table
is related to surface sediments and the values related to this study are the mean
value of the depths of 0 and 2 cm of all sediment cores. Concentration of Ni was higher
than the values of other lakes but comparable to the findings from previous studies
on Anzali wetland. Cr, Cd and Fe concentrations in this study was near to the values
of previous study on the wetland. Zn concentration was lower than the values of other
lakes and comparable to Anzali wetland previous researches. Pb concentration was lower
than previous studies on Anzali wetland and between the values of other lakes.

Table 2.Comparison of Ni, Cr, Cd, Zn, Fe and Pb concentrations in Anzali wetland and some
other water bodies

The results of heavy metal concentrations across sediment columns were compared with
the results of research carried out by Ghazban and Zare on the Anzali wetland in Table 3[17]. It can be observed that the results were relatively similar to the present study
and there was not any distinct trend for the variation of heavy metal concentrations
with the depth in sediment columns. This fact is related to the industrialization
and urbanization of the wetland basin. There are 23 large plants in the wetland basin
which only four of them have appropriate working wastewater treatment plant. Six of
the other plants do not have proper wastewater treatment system and the other plants
do not have any wastewater treatment plant [22]. Uncontrolled wastewater discharges from these plants to the rivers entering the
wetland, resulted in higher heavy metals depositing in the wetland sediments during
recent years. Whereas these pollutions do not emit to the rivers continuously, no
clear trend could be detected for heavy metal contents with the depth. Except for
industrial emission, some of these metals have considerable traffic related sources
like Pb [23]. With the rapid process of urbanization, the number of automobiles has increased
and gasoline discharges to the receiving water bodies led to heavy metals depositing
in the wetland sediments. By relating the industrialization and urbanization process
to the vertical distribution curves at these sediment cores, it is believed that the
high trace metal concentrations of sediment in Anzali wetland result from rapid urbanization
and industrialization, and lack of wastewater treatment.

To analyse the relationship between magnetic susceptibility and the concentration
of each heavy metal along the cores, the cluster analysis was applied using Pearson
correlation coefficient. Figures 9101112 show the dendrogram of magnetic susceptibility and heavy metals for core 1 to core
4 respectively. In Figure 9 there is strong relationship between MS and Ni and a relatively strong relationship
for MS with Cr and Fe in core 1. It means that magnetic properties of core 1 are related
to Ni mostly. There is also a good correlation between Pb, Zn and Cd. Figure 10 indicates a relatively strong relationship between MS and Fe in core 2, so it can
be concluded that magnetic properties of core 2 are related to Fe content. In this
core there is close correlation between Cd and Ni. Figures 11 and 12 show strong relationship between MS and Fe in core 3 and core 4. therefore their
magnetic properties are related to Fe content. There is also close correlation between
Cr, Pb and Ni in core 4. Close correlation between heavy metals signifies that they
have originated from similar contaminant sources [24].

Conclusion

The main objective of this study was to investigate the relationship between the magnetic
susceptibility and the contamination of heavy metals in sediments of Anzali wetland.
To achieve the aim, four stations in the wetland were chosen considering four different
regions of the wetland and core sediment samples were collected. Six heavy metals
(Ni, Cr, Cd, Zn, Fe, and Pb) were analyzed across each sediment core by geochemical
analysis. Whole-core magnetic susceptibility measurements were done on each sample
using Bartington MS2C System. To discover the relationship between magnetic susceptibility
and heavy metals content, cluster analysis was applied using Pearson correlation coefficient.
Major findings are listed below:

High trace metal concentrations of sediment in Anzali wetland result from rapid urbanization
and industrialization, and lack of wastewater treatment plants in surrounding industries.
Whereas pollutions haven’t emitted to the wetland continuously, no clear trend could
be detected for heavy metal contents in vertical distribution curves at these sediment
cores.

Geochemical analysis of soil samples showed different correlations of concentrations
in each core: in core 1 there was close correlation between Cd, Pb and Zn; in core
2 there was close correlation between Cd and Ni and in core 4 there was close correlation
between Cr, Pb and Ni.

Significant relationship was found to be between magnetic susceptibility and the concentration
of Fe in most of core samples. It concluded that magnetic properties of core samples
were related to Fe content.

In west part of the wetland, Abkenar region (zone A in Figure 2), the relationship between MS and heavy metals was different with the other parts
(zone B in Figure 2). It could be related to the contamination level of each zone. zone A is relatively
isolated part of the wetland (only one river inflow) and consequently is less contaminated
than zone B. comparison of average heavy metal contents in four sediment cores (Figure 3) confirmed this fact. It can be deduced from this finding that during last decades,
before urbanization and industrialization of the wetland basin, the correlation of
MS and heavy metals in Anzali wetland have been significant for Ni, Cr and Fe, but
during recent years by rapid process of urbanization and industrialization and increasing
contamination from rivers inflowing the wetland, this correlation had become significant
for Fe.

The result of this study demonstrated that magnetic susceptibility measurements could
be applied as a proxy method for heavy metal pollution determination in marine environments
in Iran especially as a rapid and inexpensive preliminary site assessment. Such a
survey should be accompanied by geochemical data for more accuracy. Although availability
of suitable cores is very important in this application, if provided, magnetic susceptibility
can offer scientists and engineers a quick and cost-effective tool of surveying seabed
contamination by heavy metals.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

The overall implementation of this study including design, experiments and data analysis,
and manuscript preparation were the results of efforts by Corresponding author. All
authors have made extensive contribution into the review and finalization of this
manuscript. All authors read and approved the final manuscript.

Acknowledgements

The authors would like to thank Iran Water Research Management Company who supported
this research as a research project (code: ENV1-89015) and Water Research Institute
who cooperated in collecting sediment samples and experimental supports.

References

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APHA AWWA, WEF: Standard methods for the examination of water and wastewater. 20th edition. Washington, DC: American Public Health Association, American Water Work Association,
Water Environment Federation; 1998.